Sub-threshold design has been widely applied owing to its ultra-low energy consumption characteristic. However, as the system supply voltage enters into the sub-threshold region, the impact of process fluctuations on the threshold voltage of MOS transistors in memory cells becomes more severe. For SRAM memory circuits, process fluctuations may result in degraded performance and even faults of the memory cells. That phenomenon brings a new challenge to stability design of the entire system. Therefore, it is necessary to employ process fluctuation compensation measures to improve stability of SRAM memory cells.
In the sub-threshold region, the threshold voltage of MOS transistor is in an exponential relationship with the driving power; therefore, the driving power of MOS transistor can be changed effectively by changing the threshold voltage of sub-threshold MOS transistor. Changing the substrate voltage of MOS transistor is one of the most effective measures for changing the threshold voltage of MOS transistor. However, owing to the fact that the substrate voltage of NMOS transistor manufactured in a single well process is the power ground, changing the substrate voltage of PMOS transistor is an effective approach to implement that method.